If you check the site below you can find a set of stamps described as 'Privately issued charity labels ...', and there is a mention of overprinting of the values.
Scroll down the LHS to Poland 1917 Polish Kings Charity Stamps.
The person on your stamp is actually Prince Jozef Poniatowski (1763-1813), but incorrectly identified on the display page as Stanislaus Poniatowski.
Not a postage stamp.
http://www.cinderellas.info/wittcol...llection.htm